Flow metering is a key measurement when attempting to optimise production from a well. However, current technologies are limited to flow measurements at a limited number of discrete locations, for example by permanent installation of optical flow meters at a number of spaced locations along a length of production tubing.
Well integrity is also a key concern. However, using such a flow metering system again only allows measurements to be made at discrete points—although by measuring the speed of sound in the tubing the contents of the tubing can be determined, albeit only at those discrete points.
Noise logging can also be employed to determine in-well fluid flow and composition. Again, such measurements can only be made at discrete points, unless they are made while lowering a hydrophone into the well. Such a measurement requires an intervention, and so is generally undesirable.
Downhole optical fibres are used in a number of different applications as a replacement for conventional technologies that cannot withstand the pressures and temperatures that fibre based sensors can withstand. Furthermore, distributed optical fibre sensors may allow simultaneous measurements at a significantly greater number of measurement points—not limited by individual physical sensors.
It is proposed by the Applicant to employ optical fibre based sensors, such as their proprietary Intelligent Distributed Acoustic Sensor (iDAS), for the purposes of wellbore surveying and in particular downhole flow metering to obtain a distributed measurement of in-well fluid flow. However, it is not obvious how the skilled person could employ the iDAS technology to produce meaningful survey data or useful distributed flow data.
It is anticipated that the solution will be applicable to many different applications and to data obtained from a variety of different measurements (i.e. not just iDAS).
It is therefore an object of at least one embodiment of the present invention to provide a method of surveying a wellbore based on obtaining a distributed acoustic measurement of the wellbore.
It is also an object of at least one embodiment of the present invention to provide corresponding methods of monitoring a formation and of monitoring fluid flow within a wellbore.